Systems for using plant matter as an energy source are well known. In the simplest case, biomass can be burned to release energy.
First-generation biofuels are made from sugar, starch or vegetable oil. The basic feedstocks for the production of first generation biofuels are typically seeds or grains such as wheat, which yield starch that is fermented into bioethanol. This process is of limited value for many reasons. For example, the feedstock could otherwise be used as food, and the energy consumed in cultivating it might in some cases exceed the energy recoverable froth the biofuel produced.
Second generation biofuel processes use biomass consisting of the residual non-food parts of current crops or non food crops, such as switchgrass, jatropha, cereals that bear little grain, and also industry waste such as wood chips. In the second generation process the plant's cellulose must be broken down to release the sugar which is then fermented to produce ethanol in the same way as first generation bioethanol production. The plant's remaining lignin can be burned as a carbon-neutral fuel to produce energy.
Second generation biofuels have been criticised because of the use of large scale agriculture to provide the feedstock, possible adverse effects on land, use of fertilisers and pesticides, risks posed by genetically engineering crops, consumption of large amounts of energy for processing and so on.
Energy can also be extracted from plant matter by pyrolysis and gasification, that is, heating under controlled conditions to produce combustible synthesis gas and oil which can be burnt or used for other purposes. Pyrolysis also produces a carbon-rich residue known as bio-char, which can be used as a soil enhancer. Bio-char also sequesters carbon for long periods, potentially thousands of years, as it is an extremely stable form of carbon.